1. Field of the Invention
Disclosed embodiments relate to a radar device and a method of processing a signal.
2. Description of the Related Art
Conventionally, there is a case where the following vehicle control is performed for a vehicle (hereinafter, referred to as a “front vehicle”) traveling on the front side of a vehicle (hereinafter, simply referred to as a “vehicle”) in which a radar device is installed. For example, there is a case where the vehicle travels with the front vehicle being set as a tracking target under the control of ACC (Adaptive Cruise Control). During performing such control, there is a case where the detection of object data of the front vehicle using the radar device is not sufficient, for example, when the vehicle travels inside a tunnel. Described in more detail, there is a case where the detection of object data corresponding to the front vehicle is not sufficient when an object (hereinafter, referred to as a “stationary object”) having a relative speed corresponding to the speed of the vehicle such as a wall disposed inside a tunnel or a guard rail is present.
Here, a process of detecting object data of the front vehicle and the stationary object will be described as below. The radar device emits a transmission wave and receives a plurality of reflected waves arrived in accordance with the reflection of the transmission wave on the stationary object and the front vehicle as a reception signal. Then, a signal processing unit of the radar device performs an FFT (Fast Fourier Transform) process based on the received signal, whereby a plurality of transformed signals are generated. Next, the signal processing unit derives peak signals exceeding a predetermined threshold out of the plurality of transformed signals, whereby a peak signal corresponding to the stationary object and a peak signal corresponding to the front vehicle are derived.
There are many stationary objects due to the periphery of the vehicle being surrounded by the walls of a tunnel inside the tunnel, and there is the influence of multipath and the like, whereby many reflected waves are received by the radar device. Accordingly, in a case where a distance between the front vehicle and a stationary object is short, in other words, in a case where a vertical distance, which is a distance corresponding to the traveling direction of the vehicle, between the front vehicle and a stationary object is relatively short, the frequency of a peak signal corresponding to the front vehicle and the frequency of a peak signal corresponding to the stationary object are frequencies that are relatively close to each other. As a result, although the peak signal corresponding to the front vehicle and the peak signal corresponding to the stationary object are originally to be separately present, the peak signal corresponding to the front vehicle is included in many peak signals corresponding to the stationary objects. In other words, the peak signal corresponding to the front vehicle according to the radar device is buried in many peak signals corresponding to the stationary objects, and there is a case where object data corresponding to the front vehicle may not be accurately detected.
Accordingly, for example, in a case where a signal processing mode of an FM-CW (Frequency Modulated Continuous Wave) is used as a signal processing mode of detection of object data in a radar device, by expanding a frequency modulation width (hereinafter, simply referred to as a “frequency modulation width”) of an UP zone and a DOWN zone corresponding to the transmission wave and the reception wave, the frequency resolution of the frequency of a peak signal derived after the FFT process can be improved. As a result, the resolution of the vertical distance of an object can be improved. Accordingly, by expanding the frequency modulation width, a peak signal corresponding to the front vehicle and a peak signal corresponding to a stationary object, which are frequencies relatively close to each other, can be separately derived.
More specifically, for example, in a case where the center frequency of the transmission signal is 76.5 GHz, in contrast to a conventional case where a frequency modulation width (the upper limit frequency is 76.6 GHz, and the lower limit frequency is 76.4 GHz) of 200 MHZ is used, for example, by expanding the frequency modulation width to 400 MHz (the upper limit frequency is 76.7 GHz, and the lower limit frequency is 76.3 GHz), the frequency resolution of the peak signal is improved, and peak signals corresponding to the front vehicle and another object that is a stationary object can be separately derived. As a material describing a technology according to this application, there is JP 2000-206241 A.
However, by expanding the frequency modulation width in this manner, another phenomenon that does not conventionally occur in an object data detecting process occurs. In other words, conventionally, even when a plurality of reflected waves reflected from the same object are received by a radar device, the distance of the reflected waves is a relatively short distance due to the reflection from the same object, and the peak signals after the FFT process have an approximately same frequency value, whereby one peak signal corresponding to the plurality of reflected waves is derived. In contrast to this, by expanding the frequency modulation width as described above, a plurality of peak signals corresponding to the plurality of reflected waves reflected from the same object are derived to have frequencies different from each other.
More specifically, for example, before the frequency modulation width is expanded, although a reception signal that is based on a reflected wave reflected from a rear bumper located on the rear side of the front vehicle and a reception signal that is based on a reflected wave reflected from a rear glass of the front vehicle that is close to the rear bumper in the vertical distance have a distance difference more or less, the reception signals are derived as one peak signal. However, by expanding the frequency modulation width, for example, one peak signal that is based on a reflected wave reflected from the rear bumper of the front vehicle and another peak signal that is based on a reflected wave reflected from the rear glass of the front vehicle are derived to have mutually different frequencies.
Here, the signal processing unit of the radar device performs a continuity determining process in which, from among a plurality of pieces of object data detected in one scanning, object data detected in this scanning within a predicted range that is based on object data (hereinafter, referred to as “past object data”) detected in the past scanning is determined as object data (hereinafter, referred to as “past correspondence data”) having time continuity with the past object data. In addition, from among a plurality of pieces of object data detected in this scanning, object data that has not been detected in the past scanning is determined as object data (hereinafter, referred to as “new data”) that is newly detected. Here, being within the prediction range that is based on the object data detected in the past scanning represents a case where pair data acquired by paring a peak signal of the UP zone and a peak signal of the DOWN zone is included within a range of predetermined values of all the parameters of the vertical distance, the relative speed, and the horizontal distance, which is a distance in the vehicle width direction with the position of the vehicle being set as the origin.
Within the new data detected in one scanning, there is object data (hereinafter, referred to as “same object data”) that corresponds to the same object as the object relating to the past correspondence data. For example, in a case where the object data of the rear bumper of the front vehicle is determined as past correspondence data, and the object data of the rear glass is determined as new data, the new data corresponding to the object data of the rear glass is the same object data.
In addition, there is a case where there is time continuity between the same object data and object data corresponding to another object in consecutively performing a plurality of scans. For example, in a case where there is object data corresponding to a vehicle (hereinafter, referred to as an “adjacent vehicle”) traveling toward the front side of the vehicle in a lane (hereinafter, referred to as an “adjacent lane”) that is adjacent to another lane in which the vehicle travels, when the vertical distance and the horizontal distance between the adjacent vehicle and the front vehicle are relatively short distances, there is a case where new data, which is the same object data, is determined as object data having time continuity with past object data corresponding to the adjacent vehicle. As a result, for example, in a case where the adjacent vehicle is present at a vertical distance shorter than that of the front vehicle from the vehicle, the tracking target may change from the front vehicle to the adjacent vehicle in the ACC control of the vehicle. Accordingly, there is a case where an object other than the object that is originally to be the tracking target is set as the tracking target, and there is a case in which safe vehicle control may not be provided to a user using the vehicle.